The present invention relates generally to heat exchangers and, more particularly, to a refrigeration system suction line/capillary tube assembly providing efficient heat exchange between the cool gaseous refrigerant conveyed by the suction line and the warm liquid refrigerant conveyed by the capillary tube. Additionally, the invention relates to a method for providing such an assembly.
It is well known in the art of refrigeration to provide heat exchange between the relatively cool refrigerant vapor conveyed from the evaporator outlet through the suction line to the compressor and the relatively warm liquid refrigerant conveyed from the condenser outlet through the capillary tube to the evaporator inlet. Such heat exchange improves the thermodynamic efficiency of the refrigeration system by cooling liquid refrigerant before it enters the evaporator. Basically, since the cold gaseous refrigerant passing through the suction line is warmed to at least ambient temperature as it passes from within the insulated space to the compressor inlet, the residual refrigeration effect of the cold suction line gas would otherwise be wasted (except for some beneficial compressor motor cooling extracted from the returning refrigerant). This heat exchange between gaseous and liquid refrigerant is often termed "suction line/capillary tube heat exchange." This terminology is for convenience used hereinafter, and is intended to mean heat exchange between the gaseous and liquid refrigerant, and not really heat exchange between sections of metallic tubing per se.
There are two prior art techniques generally used in the refrigeration art for providing a suction line/capillary tube assembly for heat exchange. The first technique is soldering the small diameter capillary tube to the outside of the suction line along a substantial length thereof to produce the suction line/capillary tube assembly. A problem with this method is that, due to the metallurgical bond required, compatability of the suction line material with the capillary tube material is a problem. Of two most commonly-employed materials, aluminum and copper, aluminum is the lower in cost. However, aluminum, while suitable for the suction line application, is generally unsuitable for the capillary tube application because the inside diameter of the capillary tube must be very small and controlled within tight tolerances. In most cases, copper, but not aluminum, is a satisfactory material for this tight tolerance application. It would therefore appear desirable to form the suction line of aluminum and the capillary tube of copper. However, forming a heat-conducting metallurgical bond between a copper and aluminum is difficult. Further, when dissimilar metals such as copper and aluminum are bonded and exposed to moisture, particularly atmospheric moisture, corrosion of the aluminum due to galvanic action occurs. The galvanic action, if not inhibited, eventually destroys the bond and the aluminum tubing. The result of all this is that where the capillary tube is soldered to the outside of the suction line, the capillary tube as well as the suction line must be copper rather than low-cost aluminum. A third possible material, steel, although low in cost, is not commonly used because of severe corrosion and handling problems.
An additional disadvantage to the first technique is a slight loss in thermodynamic efficiency due to heat loss from the capillary tube directly to the ambient. While at first it may appear that any heat lost by the capillary tube is beneficial, more heat can generally be transferred to the suction line gas than to ambient. This is because the suction line gas is normally cooler than ambient and therefore more heat is lost by the capillary tube if it is in heat exchange relationship only with the suction line gas. Additionally, in refrigerator constructions where the suction line/capillary tube assembly passes through the insulated wall space between the inner liner and the outer case, heat from the capillary tube, if not contained, can flow to the refrigerated space within the inner liner. This then adds to the work the refrigeration system must perform.
Where it is desired nevertheless to use a low-cost aluminum suction line in combination with a copper capillary tube, the second prior art technique for providing a suction line/capillary tube assembly for heat exchange is to pass the copper capillary tube inside the suction line in heat exchange relationship with the gaseous refrigerant passing therethrough. In the art, this is generally termed a "coaxial" heat exchange, although actually the capillary tube is not necessarily precisely centered within the suction line. The capillary tube may even contact the interior surface of the suction line at random points. Galvanic corrosion is not a problem because all points of contact between aluminum and copper are within the sealed refrigerant system and therefore not exposed to atmospheric moisture.
While the coaxial heat exchange effectively eliminates the material compatability problem and eliminates the problem of heat leakage from the capillary tube to ambient and to the cool interior of the refrigerator cabinet, the heat exchange between the capillary tube and gaseous refrigerant carried by the suction line is not as efficient as in the first method because the much greater heat exchange surface provided by using the interior wall of the suction line itself to heat exchange with the gaseous refrigerant is largely lost.
By the present invention, there is provided a suction line/capillary tube assembly which provides improved heat exchange efficiency and which permits the use of a low-cost aluminum suction line.